In addition to telluric contamination, the data contain many photospheric absorption lines, typical of a K7 star. Some of these were blended with the [Oi] 6300 Å line and required removal to study its profile. Unlike the telluric absorption lines, it was possible to remove all the photospheric lines in this region (\(6284 - 6342\)  Å) at once. This was done by obtaining a WTTS spectrum of the same spectral type as a template, which contains the same photospheric lines but no emission lines [1]}, and subtracting this from the CTTS spectrum of RU Lup. As WTTSs undergo little-to-no accretion activity (which produces many emission lines), their spectra can be used as templates for the photosphere of CTTSs of the same spectral type.
Indeed, the RU Lup spectra was compared with the WTTS spectrum of TAP 45 [2]}. TAP 45 has a stellar mass of \(\sim \!0.79\,M_{\odot }\) , a radius of \(\sim \!1.14\,R_{\odot }\)  and \(T_{\text{eff}}\sim \,4040\)  K [3]}. This template was then re-scaled such that it met the level of the RU Lup continuum; these parameters were adjusted by minimising a \(\chi ^2\)  fit. The resultant broadened and veiled template represents a spectrum containing the fitted photospheric features which can then be used to subtract the contaminating photospheric lines from the regions of interest. This process was completed on a night-by-night basis.
